Welding plastic tube lining

ABSTRACT

Welding apparatus for plastic lining (5) in concrete tubing (4) comprises a centering carriage (6) movable inside the tubing along its length and fixable in position at a location adjacent a butt joint (51) having a seam to be welded. A carrier arm (10) is mounted for rotation about an axis (9) coincident with the axis of the tubing (4) for supporting a welding unit (14) at its outer end remote from the axis of the tubing. A drive source (15, 16) adjacent the welding unit engages the interior of the tubing and self propells the welding unit (14) and carrier arm (10) around the interior of the tubing (4). The welding unit (14) is biased outwardly towards the wall of the tubing and is guided along the welding seam by either a guide ring (21) or a roller (20) engaging a gap (48) between adjacent tubing sections. Several degrees of freedom are provided for accommodating lack of concentricity and translation of the welding unit along the length of the tubing without moving the centering carriage.

FIELD OF THE INVENTION

This invention concerns welding of the plastic lining inside concretetubing at the joint between adjacent sections of tubing.

BACKGROUND

Some types of concrete tubing such as those intended for use as sewerpipe are provided with plastic lining for protecting the concrete fromthe effects of material in the pipe. In exemplary embodiments the liningis polyvinyl chloride or other thermoplastic material in the order oftwo to five millimeters thick. Such tubing may be lined around theentire circumference or around an upper portion of the circumference.The latter embodiment is suitable for sewer pipe where a lower portionof the pipe is protected by immersion in liquid contents of the pipe andonly an upper portion is exposed to gases that could damage theconcrete.

In either embodiment it is important to seal the plastic lining withinadjacent sections of tubing at the butt joint between the sections. Insome embodiments a flap of extra plastic is provided at one end of eachsection of tubing and that flap is lapped over the plastic lining in anadjacent section of pipe and sealed in place. Alternatively, a strip ofplastic can be added along the joint overlapping the plastic lining inadjacent sections of tubing. In either embodiment the overlapped portionis preferably heat welded for providing a fluid-tight seal.

It has been the practice to manually weld such plastic linings using ahot air gun or the like for melting the plastic. Such techniques arecostly and difficult and can be subject to variable quality. It istherefore desirable to provide a technique for rapidly and reliablywelding the joints in plastic lining in concrete tubing or the like.Preferably the technique should be suitable for tubes of a variety ofdiameters and for either lap joints employing a flap of plastic from onetubing section overlapping the plastic in another section or jointsemploying an added sealing strip of plastic over the lining in adjacentsections. Preferably the technique is suitable for welding the liningadjacent a butt joint normal to the tubing axis or at a beveled ormitered joint adjacent a change in direction of the tubing.

BRIEF SUMMARY OF THE INVENTION

There is, therefore, provided in practice of this invention an apparatusfor welding the plastic lining of tubing comprising a carriage movableinside such tubing along its length. In use the carriage is fixed inposition within the tubing at a selected location adjacent a seam to bewelded. At least one carrier arm is mounted on the carriage for rotationabout an axis coinciding with the axis of the tubing and a welding unitis mounted on an outer end of such a carrier arm remote from the axis ofthe tubing so as to be near the wall of the tubing. A drive sourceadjacent the welding unit includes means for engaging the interior ofthe tubing and self propelling the welding unit and carrier arm aroundthe interior of the tubing. The welding unit and drive are biasedoutwardly toward the wall of the tubing, and are guided along a weldingseam around the tubing, either by a guide wheel in the gap betweenadjacent pipe ends or by guide rollers engaging a guide ring temporarilyclamped in the tubing. Translation of the welding unit along the lengthof the tubing to remain aligned with the welding seam is accommodatedwithout moving the carriage. Such translation can be accommodated bymounting the welding unit on a pivot arm which can also pivot about anaxis extending radially in the tubing. The carrier arm can also bemounted for pivoting about an axis normal to the tube axis so that thewelding unit can be guided along a bevel joint.

DRAWINGS

Additional features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 is a semi-schematic end view of welding apparatus constructedaccording to principles of this invention within a tube illustrated intransverse cross section;

FIG. 2 is a side view of the apparatus of FIG. 1;

FIG. 3 is a detailed side view of the welding unit and associated guidecarriage of the apparatus of FIG. 1;

FIG. 4 illustrates an alternative means for mounting the welding unit;

FIG. 5 illustrates an exemplary overlap joint;

FIG. 6 illustrates an exemplary overlap joint with a welding strip;

FIG. 7 illustrates an exemplary butt joint sealed with a sealing strip;

FIG. 8 illustrates an exemplary butt joint employing a sealing stripwith a welding strip at each edge thereof;

FIG. 9 illustrates semi-schematically a detail of a guide roller forguiding the welding unit along a welding seam;

FIG. 10 is a semi-schematic side view of another embodiment of liningwelding apparatus in a longitudinal cross section of tubing;

FIG. 11 is a fragmentary detail of mounting for a carrier arm on theapparatus of FIG. 10;

FIG. 12 is a semi-schematic end view of the apparatus of FIG. 10 in atransverse cross section of tubing; and

FIG. 13 is a detailed view of pressure rollers of the apparatus of FIG.10 adjacent a butt joint.

DESCRIPTION

FIGS. 1 to 3 illustrate an apparatus for welding a joint betweenabutting tubes 4, particularly concrete tubes having a plastic lining 5.Types of joints that can be welded are illustrated in cross section inFIGS. 5 to 8. Thus, FIG. 5 illustrates an overlap joint 1 where theplastic lining 5 in one of the sections of tubing 4 includes a flap 36which overlaps the plastic lining in an adjacent section of tubing at abutt joint. A weld 37 is made between the overlapping flap 36 and theplastic lining 5 by heating the plastic to its welding temperature andpressing the two layers of plastic together.

FIG. 6 illustrates an overlap joint wherein a flap 36 is welded to thelining of an adjacent tubing section with use of a welding strip 3overlapping the edge of the flap and the adjacent tubing lining.

FIG. 7 illustrates a sealing strip joint in which a sealing strip 2 isapplied along a butt joint between adjacent tubing sections so as tooverlap the tubing lining 5 in each of the tubing sections. The sealstrip is welded to the underlying pipe linings at welds 38 along eachedge.

FIG. 8 illustrates a butt joint having a sealing strip 2 similar to thejoint of FIG. 7 with addition of a welding strip 3 along each edge ofthe seal strip. Apparatus as provided in practice of this invention canbe employed for forming any of these types of joints.

Referring again to FIGS. 1 to 3 the apparatus comprises a centeringcarriage which is movable within the tubes 4 along their length by meansof casters 7. The height of at least a portion of the casters 7 can beadjusted by threaded shafts and hand wheels 39 for accommodatingdifferent pipe diameters. An adjustable locating prop 8 can be raisedinto engagement with the upper wall of the tubing for fixedlypositioning the centering carriage in the tubing adjacent a seam to bewelded. Such adjustments permit the apparatus to be used in tubinghaving inside diameters ranging from one to one and one-half meters. Asomewhat larger apparatus similar in principle can be adjusted for pipediameters in the range of one and one-half to three and one-half meters.

A pair of carrier arms 10 are mounted on the centering carriage 6 forrotation about an axis 9 coincident with the axis of the tubing 4. Theaxis of rotation 9 can be adjusted into coincidence with the tubing axisby the adjustable wheels 7 and prop 8 and once adjusted the apparatuscan be used for making repeated welded joints in tubing of the samesize.

At the end of each carrier arm 10 a transverse pivot arm 12 is mountedon a fork member 22 so that the pivot arm is free to pivot around anaxis 11 which extends more or less parallel to the tube axis 9 (subjectto non-alignment due to the degrees of freedom available in theapparatus). A conventional welding unit 14 is mounted on a guidecarriage 20 at one end of the pivot arm 12. The welding unit 14illustrated in the enlarged detail of FIG. 3 comprises a conventionalhot air blower fitted with a right angle nozzle 41 which directs heatedair underneath a flap or welding strip for heating plastic to itsmelting point and welding the joint.

The welding unit includes an adjustable speed drive source 15 for atleast one drive roller 16 engaging the interior wall of the tubing.Thus, the welding unit is a self-propelled device with the drive source15 and drive roller 16 acting as the rotary drive source for the carrierarm 10. Such a combined drive and welding unit is commerciallyavailable.

A pressure roller 17 on the welding unit compresses the welded jointafter the plastic material is heated to its welding temperature. Thisassures proper compression of the lining material to be welded andcontinuous welding thereof in the overlapped region. By having theadjustable speed, self-propelled drive source engage the wall surface ofthe tubing, welds can be made in a broad variety of pipe diameters withlimited speed adjustment since surface speed of the welding unit alongthe welding seam is closely controlled without regard to the angularvelocity of the carrier arms 10 around the tube axis 9. Further, thetorque required of the drive motor is less than would be needed for amotor driving the carrier arms directly. It is desirable to employ twodrive sources 15, i.e, at the outer end of each of the carrier arms forenhanced reliability. Thus, if one drive wheel were to skid on theplastic lining, the other drive wheel can continue to rotate the weldingunits around the axis 9. Preferably two drives are used even if only onewelding unit is active.

A slip ring housing 35 is interposed between the mounting for thecarrier arms 10 and the centering carriage 6 for permitting electricalenergy to be supplied to the welding unit 14 and drive means 15.

In addition to being pivotable about the axis 11 extending parallel tothe tube axis 9 and spaced eccentrically thereform, the pivot arm 12 ismounted for pivoting about a tube radial axis 19 extending along thelength of the carrier arm 10. The fork member 22 has a shaft portion 23extending inside the hollow carrier arm 10 for a distance limited by astop collar 24 engaging the end of the tubular carrier arm 10. The shaftis free to rotate within the carrier arm.

An angular mounting bracket 25 on the fork member supports a tensioningdevice 13 which resiliently biases the welding unit 14 and guidecarriage 20 towards the tubing wall. In this embodiment the means forbiasing comprises a pair of tension springs 26 connected to the oppositeend of the pivot arm 12 from the guide carriage 20. An adjustablespindle 28 connected to the tension springs 26 is in threaded engagementwith a spindle nut 27 on the mounting bracket 25. The adjusting spindle28 permits the operating pressure of the welding unit 14 and hence thepressure roller 17 to be adjusted for assuring a sound weld.

The guide carriage 20 is also connected to the end of the pivot arm 12for rotation about an axis extending generally radially with respect tothe axis of the tubing. In the illustrated embodiment the guide carriage20 is connected to the pivot arm 12 by a ball and socket joint 31 or itsequivalent having at least two degrees of freedom of pivoting. The guidecarriage 20 has guide rollers 18 mounted thereon by means of adjustablelength legs 34. The guide rollers 18 engage the edges of a guide ring 21so that the guide carriage and welding unit including the pressureroller 17 accurately follow the seam being welded. The guide ring isremovably clamped in position inside the tubing adjacent the weldingseam at each joint to be welded. Pilot rollers 33 also maintain theguide carriage in proper position relative to the tubing wall.

Thus, the guide carriage and welding unit have several degrees offreedom so as to accurately and reliably follow a guide ring alignedwith a welding seam. The ball joint 31 permits the carriage to skew asguided by the rollers 18 and permits tilting relative to the tubing wallas regulated by the drive roller 16, pressure roller 17 and pilotrollers 33. Rotation of the pivot arm 12 about the generally radial axis11 accommodates translation of the guide carriage along the length ofthe tubing as may be needed, for example, in case the centering carriage6 is out of position along the length of the tubing. Further, pivotingof the pivot arm 12 about the generally axially extending axis 11 incombination with the ball joint mounting of the guide carriage and thebiasing device 13 maintains the guide carriage adjacent the tubing wallin case the pivot axis 9 is somewhat out of coincidence with the tubingaxis or the tubing is out-of-round. Pivoting of the pivot arms about theradial axis 19 permits the guide carriages to track a somewhat diagonaljoint at a mitered corner between adjacent pipe sections. Thus, eachpossible joint configuration and error in positioning the centeringcarriage is accommodated by the degrees of freedom provided in mountingthe guide carriage.

In using the apparatus for welding a butt joint where adjacent sectionsof tubing are aligned, the axis of rotation 9 substantially coincideswith the common axis of the two sections of tubing. The carrier arms 10extend essentially radially in such tubing. When the apparatus isemployed for welding a beveled or mitered butt joint where the axes ofadjacent sections of tubing intersect at an angle, such as, for example5°, the axis of rotation 9 of the carrier arms is normal to the plane ofthe joint and the carrier arms rotate essentially in the plane of thejoint. For purposes of this specification the axis of rotation 9 isstill considered coincident with the axis of the tubing since it isessentially at a mean position between the intersecting axes of theadjacent sections of tubing. Similarly the axis of rotation of the pivotarm is considered to extend radially. It will be recognized that in suchan embodiment the welding unit or units travel along an elliptical pathwithin the tubing rather than a circular path as in a straight buttjoint. The differences in distance between the tubing axis and the wallare accommodated by the means for biasing the welding unit outwardly onthe pivot arm.

In the illustrated embodiment the carrier arms 10 with their associatedpivot arms 12, tensioning devices 13, welding units 14, and guidecarriages 20 are essentially identical on both sides of the axis ofrotation 9, thereby permitting two welded seams to be finished during asingle revolution of the apparatus, such as, for example, in a jointusing a sealing strip. Such an arrangement also enhances the balance ofthe rotating portion of the apparatus. When only one seam is beingwelded, such as in an overlap joint, one of the welding units can beinactive although it is desirable to operate both drive sources 15 forbest reliability.

When the apparatus is used, a guide ring 21 is properly aligned andclamped in place within the tubing adjacent a seam to be welded. Thecentering carriage 6 is advanced to a position substantially asillustrated in FIGS. 1 and 2, and the props 8 are elevated for fixingthe centering carriage in the tubing. The guide carriage or carriages 20are positioned adjacent the tubing wall with the guide rollers 18engaging the edges of the guide ring 21. The nozzle 41 of the weldingunit 14 is placed into the seam to be welded for heating the fayingsurfaces.

The welding unit and drive source 15 are started so that plastic in theseam is heated to the welding temperature. The guide carriage isadvanced by means of the driven drive roller 16 and pressure roller 17.The pressure roller applies pressure to the heated plastic and assures asound weld as the guide carriage is driven around the interior of thetubing. When the welding seam is completed, the guide carriage andwelding head are retracted from the tubing wall and latched by a lockingspindle 32 and cable 42. The guide ring can then be unclamped and movedto a new position adjacent the next seam to be welded. Additionaldegrees of freedom of the apparatus are locked by means of latchingarrangements (not shown), the props are retracted, and the centeringcarriage is rolled to the next seam to be welded.

In many embodiments a lower sector of the pipe is unlined and the weldis commenced and ended adjacent the unlined portion. In an embodimentwhere the entire tubing circumference is lined, the apparatus is usedaround almost all of the circumference and the final short sector iswelded by hand.

If desired for making a weld having a sealing strip 2 as illustrated inFIGS. 7 and 8 and/or employing a sealing strip 3 as illustrated in FIGS.6 and 8, rolls of such material can be mounted on the guide carriage 20for continuous application to the seam being welded. In the case ofwelding a sealing strip joint both edges of the sealing strip can besimultaneously welded by coupling an additional welding unit adjacentwelding unit 14 on one carrier arm at an appropriate lateral spacingcorresponding to the width of the sealing strip.

In some embodiments, particularly for a joint employing a sealing stripas illustrated in FIGS. 7 and 8, a guide ring clamped in the plasticlined tubing may not be needed. A guide roller mounted on the guidecarriage and fitting into the gap of the butt joint between tubingsections can maintain the welding unit in proper alignment with a seambeing welded. Such a guide roller is illustrated in FIG. 9.

In this embodiment the guide roller 46 has an oval or ellipticalcross-sectional shape surrounded by a guide flange 47. The guide flangeis somewhat narrower than the minimum gap 48 at the butt joint betweenadjacent tubing sections 4 illustrated in phantom in FIG. 9. When thegap between adjacent sections of tubing is larger than the minimum, theoval or elliptical cross-sectional shape permits the guide roller 46 topenetrate more or less deeply into the butt joint so that properengagement therewith is assured at all times even if an angulardisplacement or gaping of the butt joint has to be accommodated. Such aguide roller arrangement can maintain the guide carriage in alignmentwith a welding seam as effectively as the guide roller 18 engaging aguide ring 21 as illustrated in FIGS. 1 and 2. If desired, the guideroller 46 can have a cross section in the form of double cones with aradial guide flange between the bases of the cones.

FIG. 4 illustrates in fragmentary detail another mounting for a weldingunit on a centering carriage particularly useful for small diametertubing. In this embodiment the carrier arm 10 is mounted for rotationabout the tubing axis 9 as hereinabove described. A pivot 77 is providednear the tubing axis 9 for pivoting the carrier arm 10 about an axis 40perpendicular to the tubing axis 9. In the illustrated embodiment thepivot axis 40 is offset from the axis of rotation 9, however, if desiredthese axes can intersect.

The carrier arm 10 is tubular with an internal stop shoulder 78 at itsouter end. A hollow shaft 79 has a portion inserted into the tubularcarrier arm 10 for translation within the carrier arm. An axial guidetube 80 extends inwardly from the outer end of the shaft inside thetubular carrier arm 10 and a compression spring 81 is positioned aroundthe guide tube 80 between the stop shoulder 78 and a flange 82 on theend of the guide tube. The shaft 79 is free to rotate within the tubularcarrier arm 10 thereby providing a degree of rotational freedom about anaxis 82 perpendicular to the pivot axis 40. The spring 81 resilientlybiases the shaft 79 outwardly towards the wall of the tubing. The flange82 is internally threaded and engaged by a threaded shaft 84 connectedto a hand wheel 85. This permits adjustment of the compression on thespring 81 and hence adjustment of the force applied to a guide carriage20. The guide carriage 20 for a welding unit (not shown in FIG. 4) ismounted on the outer end of the shaft 79 by a ball and socket joint 83which provides at least two degrees of rotational freedom perpendicularto the axis 82 of the shaft 79.

The several degrees of freedom permit the welding unit and guidecarriage to follow a welding seam. The pivot axis 40 for the carrier armpermits translation of the welding unit along the length of the tubing.The degrees of freedom provided by the ball and socket joint 83 keep theguide carriage parallel to the tubing wall regardless of pivoting aroundthe carrier arm pivot axis 40 or deviations from concentricity of theaxis of rotation 9 relative to the axis of tubing. Rotation of the shaft79 in the tubular carrier arm 10 permits skewing of the guide carriagefor following a guide ring. The thrust of the spring 81 resilientlybiases the welding unit towards the tubing wall. Such an arrangement canbe employed for welding small angle mitered joints by reason of pivotingof the carrier arm around the pivot axis 40.

In the illustrated embodiment the pivot axis 82 is offset from the axisof rotation 9 of the carrier arm. Such is convenient for fittingapparatus in small diameter tubing. In apparatus for larger tubing it isconvenient to have the pivot axis 40 for the carrier arms and the pivotaxis 82 for the guide carriage both extend radially in the tubing.

FIGS. 10 to 13 illustrate semi-schematically another embodiment ofapparatus for welding the plastic lining of concrete tubing or the like.The apparatus illustrated in these drawings straddles a butt joint 51between adjacent tubes 52, particularly large diameter concrete tubesprovided with a plastic lining 53. The apparatus comprises two similarmovable centering carriages 54 inside the tubing 52. The centeringcarriages are mounted on casters 55 bearing against a lower portion ofthe tubing for moving the carriages along the length of the tubing. Thecentering carriages can be temporarily coupled to each other by acoupling rod 56 for transport purposes. When the centering carriages areproperly positioned in the sections of tubing straddling a butt joint 51they are fixed in position by adjustable props 57 which clamp against anupper portion of the tubing and temporarily fix the centering carriagesin position. A pair of catwalks 75 are provided along the length of thecentering carriages in embodiments for large size tubing for providingaccess by workmen.

A roll carrier 58 is mounted on the centering carriages for rotationabout an axis 59 coinciding with the axis of the tubing. Two generallyC-shaped carrier arms 60 connected with counterbalance arms 61 areconnected to the roll carrier 58 so that they can also rotate about theaxis 59 coincident with the axis of the tubes.

The roll carrier 58 and carrier arms 60 are supported on the axis 59 byway of a Cardan shaft 76 to compensate for non-alignment of the twocentering carriages and keep rotation of the two carrier armssynchronized. A drag brake 30 is connected to the Cardan shaft forproviding a resisting torque so that the carrier arms can be resilientlybiased towards the tubing wall.

A welding unit 62 is mounted at the end of each of the carrier arms 60adjacent the wall of the lined tubing. The welding units 62 on the twocarrier arms are spaced apart a predetermined distance for forming twowelding seams. A pair of pressure rollers 63 is mounted adjacent thewelding unit 62 straddling the butt joint 51 for applying pressure towelded seams.

A sealing strip roll 64 is mounted on the roll carrier for feeding asealing strip 65 towards the butt joint 51 for overlapping both edges ofthe butt joint. A pair of welding strip rolls 66 on the roll carrier 58feed welding strips 67 towards the tubing walls for overlapping oppositeedges of the sealing strip 65 and the adjacent pipe lining 53.

The carrier arms are mounted on a portion of the roll carrier 58 forlimited pivotal movement about a pivot axis 68 parallel to the rotationaxis 59 and eccentrically spaced therefrom, and for limited axialmovement along the length of the tubing along the pivot axis 68. Anadjustable counterbalance 69 on each counterbalance arm 61 minimizesdeflection of the carrier arms as they rotate about the tubing axis 59.The carrier arms 60 are biased for pivoting around the pivot axis 68towards the wall of the tubing by a tension spring 70 connecting thecounterbalance arm 61 with the roll carrier 58. The tension springbiases the pressure rollers 63 towards the wall of the tubing forapplying a predetermined force to the plastic in the seams being welded.

A pivot lever 71 is also mounted between the carrier arms 60 andsupports a guide roll 46 as illustrated in FIG. 9. The guide roll 46 ismaintained in resilient engagement with the butt joint 51 by a tensionspring 62 connected between the roll carrier 58 and the pivot lever 71.

The weld units 62 are driven around the inside of the tubing byadjustable speed driving means 73 mounted on each carrier arm. The drivemeans operate drive wheels 74 which engage the wall of the tubing anddrive the welding units in a manner as hereinabove described in thedirection indicated by the arrow in FIG. 12.

As the roll carrier and carrier arms rotate about the tubing axis 59, asealing strip 65 and welding strips 67 are fed from the respective rollstoward the wall of the tubing overlapping the butt joint. Hot air fromthe welding units 62 is directed onto the faying surfaces of the weldingstrips and plastic lining of the tubing thereby heating them to theirwelding temperature. The welding strips are engaged by the pressurerollers 63 which apply sufficient pressure to assure a sound weld. Thewelding units are guided around the tubing by the guide rollermaintained in the gap of the butt joint by the tension spring 72.Pressure is maintained on the pressure rollers by the tension spring 70.Any lack of concentricity of the centering carriages is compensated forby pivoting of the carrier arms about the eccentric pivot axis 68.Variations in position along the length of the tubing are compensatedfor by longitudinal motion of the carrier arms along the pivot axis 68.

An apparatus as described and illustrated in FIGS. 10 to 13 can also beemployed for welding an overlapped joint as illustrated in FIGS. 5 and6. When operated in this mode only one-half of the welding apparatusshown is required and the sealing strip is deleted. In such anembodiment an auxiliary guide ring is temporarily clamped in properalignment on the interior wall surface of one of the sections for tubingfor positively guiding the welding unit.

What is claimed is:
 1. Apparatus for welding plastic lining of tubingcomprising:a carriage movable along the length of such tubing insidesuch tubing; means for fixing position of the carriage at a selectedlocation in the tubing; at least one carrier arm mounted on the carriagefor rotation about an axis coinciding with the axis of such tubing; awelding unit for welding plastic tubing lining mounted on an outer endof such a carrier arm remote from the axis of rotation; drive means forpropelling the welding unit and carrier arm around the interior of suchtubing; means for resiliently biasing the welding unit outwardly towardsthe wall of the tubing; guide means adjacent the welding unit forguiding the welding unit along a welding seam around the tubing; andmeans for accommodating translation of the welding unit along the lengthof the tubing in response to the guide means without moving thecarriage.
 2. Apparatus as recited in claim 1 wherein the means foraccommodating translation comprises:a pivot arm between the outer end ofthe carrier arm and the welding unit for mounting the welding unit; andmeans interconnecting the pivot arm and the carrier arm foraccommodating rotation of the pivot arm about an axis extending radiallyin such tubing.
 3. Apparatus as recited in claim 2 comprising aconnection between the pivot arm and welding unit having at least twodegrees of rotational freedom.
 4. Apparatus as recited in either ofclaims 2 or 3 wherein the means for biasing comprises a tension springconnected to the opposite end of the pivot arm from the welding unit anda pivot between the ends of the pivot arm having a pivot axis extendingparallel to the axis of rotation.
 5. Apparatus as recited in claim 1comprising means for pivoting the welding unit about an axis transverseto the axis of rotation.
 6. Apparatus as recited in claim 1 comprising apivotal connection between the carrier arm and the welding unit havingat least two degrees of rotational freedom.
 7. Apparatus as recited ineither of claims 5 or 6 wherein the means for biasing comprises a springdirecting force outwardly along an axis extending outwardly in suchtubing.
 8. Apparatus as recited in claim 1 wherein the guide meanscomprises a plurality of rollers adjacent the welding unit for engaginga ring temporarily mounted in such tubing adjacent a welding seam. 9.Apparatus as recited in claim 1 further comprising means for pivotingthe carrier arm about an axis perpendicular to the axis of rotation. 10.Apparatus as recited in claim 8 comprising means for pivotally mountingthe welding unit on an outer end of the carrier arm for pivoting aboutan axis perpendicular to the axis about which the carrier arm can pivot.11. Apparatus as recited in claim 8 wherein the means for biasingcomprises a spring directing force outwardly along the pivot axis forthe welding unit.
 12. Apparatus as recited in any of claims 9, 10 or 11comprising a pivotal connection between the carrier arm and the weldingunit having two additional degrees of rotational freedom.
 13. Apparatusas recited in claim 9 wherein the guide means comprises a plurality ofrollers adjacent the welding unit for engaging a guide ring temporarilymounted in such tubing, and further comprising a ball and socket jointbetween the welding unit and the carrier arm.
 14. Apparatus as recitedin claim 1 wherein the drive means is mounted adjacent the welding unitand comprises means for engaging an interior wall of the tubing forself-propelling the welding unit and carrier arm around the axis ofrotation.
 15. Apparatus as recited in claim 1 wherein the mounting forthe welding unit on the carrier arm comprises a tube member and a shaftmember inserted in the tube member for rotation about an axis extendingtransverse to the axis of rotation.
 16. Apparatus as recited in claim 15wherein the means for biasing comprises a spring between the tube memberand the shaft member for exerting a biasing force outwardly. 17.Apparatus as recited in claim 1 further comprising a pivot arm, meansfor mounting the pivot arm on the carrier arm for pivotal movement abouta pivot axis extending transverse to the axis of rotation of the carrierarm, and means for mounting the welding unit on the pivot arm. 18.Apparatus as recited in claim 17 wherein the welding unit is pivotallymounted on the pivot arm with at least two degrees of rotational freedomtransverse to the pivot axis.
 19. Apparatus as recited in claim 1wherein the guide means comprises a plurality of guide rollers forengaging a guide ring temporarily mounted in such tubing.
 20. Apparatusas recited in claim 1 wherein the guide means comprises a guide rollerfor fitting in the gap of a butt joint between adjacent sections oftubing and means for resiliently biasing the guide roller into such gap.21. Apparatus as recited in claim 20 further comprising means forapplying a plastic sealing strip along such a butt joint.
 22. Apparatusas recited in claim 1 wherein such carriage comprises a plurality ofadjustable wheels on the carriage for engaging an interior wall surfaceof such tubing at its lower portion and the means for fixing positioncomprises at least one height adjustable prop for temporary engagementwith an interior wall surface of such tubing at its upper portion. 23.Apparatus as recited in claim 1 wherein the carriage comprises at leasttwo carriages interconnected at the axis of rotation and the carrier armis mounted between such carriages.
 24. Apparatus as recited in claim 1comprising a roll carrier for applying a sealing strip overlapping awelding seam and at least one pressure roller adjacent the welding unitfor pressing an edge of the sealing strip against a plastic lining ofsuch tubing.
 25. Apparatus as recited in claim 1 comprising means formounting the carrier arm for pivoting about an axis parallel to the axisof rotation and eccentrically spaced therefrom.
 26. Apparatus as recitedin claim 25 wherein the means for mounting the carrier arm includeslimited movement of the carrier arm along the pivot axis.
 27. Apparatusas recited in claim 1 wherein the welding unit comprises a hot airblower for heating plastic lining to its welding temperature and atleast one pressure roller for pressing the welding seam.
 28. Apparatusfor welding plastic lining of tubing comprising:at least one carriagemovable along the length of such tubing inside such tubing; at least onecarrier arm mounted on such a carriage for rotation about an axiscentrally located in such tubing; a welding unit for welding plasticlining of such tubing; means for mounting the welding unit on thecarrier arm remote from the axis of rotation and pivotable about a pivotaxis transverse to the axis of rotation; drive means for propelling thewelding unit around at least a portion of the circumference of suchtubing; means for biasing the welding unit outwardly toward the wall ofsuch tubing; and guide means adjacent the welding unit for guiding thewelding unit along a welding seam around such tubing.
 29. Apparatus asrecited in claim 28 comprising a pivotal connection between the weldingunit and carrier arm having at least two degrees of rotational freedomtransverse to the pivot axis.
 30. Apparatus as recited in claim 29wherein the means for biasing comprises a spring directing forceoutwardly along the pivot axis.
 31. Apparatus as recited in any ofclaims 28, 29 or 30 comprising means for mounting the carrier arm forpivoting about an axis perpendicular to the axis of rotation. 32.Apparatus for welding plastic lining of tubing comprising:at least onecarriage movable along the length of such tubing inside such tubing; atleast one carrier arm mounted on such a carriage for rotation about anaxis coinciding with the axis of such tubing; means for pivoting thecarrier arm about an axis perpendicular to the axis of rotation; awelding unit for welding plastic lining of such tubing; means forpivotally mounting the welding unit on an outer end of the carrier armremote from the axis of rotation with at least two degrees of rotationalfreedom; drive means for propelling the welding unit and carrier armaround the interior of such tubing; means for biasing the welding unitoutwardly toward the wall of such tubing; and guide means for guidingthe welding unit along a welding seam around such tubing.
 33. Apparatusas recited in claim 32 wherein the means for biasing comprises a springdirecting force outwardly along an axis extending radially in suchtubing.
 34. Apparatus as recited in either of claims 32 or 33 comprisingin addition means for pivotally mounting the welding unit or an outerend of the carrier arm for pivoting about an axis perpendicular to theaxis about which the carrier arm can pivot.
 35. Apparatus as recited ineither of claims 32 or wherein the guide means comprises a plurality ofrollers adjacent the welding unit for engaging a ring temporarilymounted in such tubing adjacent a welding seam.
 36. Apparatus as recitedin either of claims 32 or 33 wherein the drive means is mounted adjacentthe welding unit and comprises means for engaging an interior wall ofthe tubing for self-propelling the welding unit and carrier arm aroundthe axis of rotation.
 37. Apparatus as recited in claim 32 comprising inaddition a tube member and a shaft member inserted in the tube memberfor rotation about an axis transverse to the axis about which thecarrier arm can pivot.
 38. Apparatus for welding plastic lining oftubing comprising:a carriage movable along the length of such tubinginside such tubing; at least one carrier arm mounted on the carriage forrotation about an axis centrally located in such tubing; a welding unitcomprising a hot air blower for heating plastic tubing lining and apressure roller for pressing a welding seam; means for mounting thewelding unit on an outer end of such a carrier arm remote from the axisof rotation; drive means adjacent the welding unit for engaging aninterior wall of such tubing and propelling the welding unit and carrierarm around the interior of such tubing; means for resiliently biasingthe welding unit and drive means outwardly towards the wall of thetubing; and guide means adjacent the welding unit for guiding thewelding unit along a welding seam around the tubing.
 39. Apparatus asrecited in claim 38 wherein the means for mounting the welding unitcomprises means interconnecting the welding unit and the carrier arm foraccommodating rotation of the welding unit about an axis extendingoutwardly in such tubing.
 40. Apparatus as recited in claim 39comprising an additional connection between the pivot arm and weldingunit having at least two degrees of rotational freedom.
 41. Apparatus asrecited in either of claims 39 or 40 wherein the means for biasingcomprises a spring directing force outwardly along the axis extendingoutwardly in such tubing.
 42. Apparatus as recited in claim 41 whereinthe guide means comprises a plurality of rollers adjacent the weldingunit for engaging a ring temporarily mounted in such tubing adjacent awelding seam.
 43. Apparatus as recited in any of claims 38, 39, or 40further comprising means for pivoting the carrier arm about an axisperpendicular to the axis of rotation.
 44. Apparatus for welding plasticlining of tubing comprising:at least one carriage movable along thelength of such tubing inside such tubing; at least one carrier armmounted on such a carriage for rotation about an axis centrally locatedinside such tubing; a pivot arm mounted on the carrier arm for pivotingabout a pivot axis transverse to the axis of rotation; a welding unitfor welding plastic lining of tubing mounted on the pivot arm offsetfrom the pivot axis; means for biasing the welding unit outwardly towardthe wall of such tubing; drive means for propelling the welding unitaround at least a portion of the circumference of such tubing; and guidemeans for guiding the welding unit along a welding seam around suchtubing.
 45. Apparatus as recited in claim 44 comprising means formounting the welding unit on the pivot arm with at least two degrees offreedom of rotation about axes offset from the pivot axis and the axisof rotation.
 46. Apparatus as recited in either of claims 44 or 45comprising means for mounting a center portion of the pivot arm forpivoting about an axis normal to the pivot axis and wherein the meansfor biasing comprises an adjustable tension spring connected to an endof the pivot arm opposite the end of the pivot arm on which the weldingunit is mounted.